I am far from an Audio Pro, yet have been trying to take some old cassette tapes and restore the sound quality they originally had, and enhance it even. As for this file in particular it seems to have lost its high's and mids. When I run it through a simple EQ plugin I just dont get the clarity I need, it just sounds worse.

Here is the steps I took in recording this tape.

#1 - Used a brand new Sony Tape deck , with preamp mixer and got the sound quality as close to 0 DB as I could (as reccomended by many audio recording sites I have read). I recorded at 44.1 hz, 16 bit through my SBLIVE card. I saw no loss in quality from tape to digital.

#2 - I then loaded Wavlab and watched the spectrum analyser and saw the highs and some mids were very low. I tried to raise this up in value but it just made the sound even more distorted than it already was.

I have many tapes like this I would like to restore, re-master (or whatever the term is). I have attached this file (in original .wav format) and uploaded it to the alt.binaries.sound.mp3 newsgroup. The topic is called:

"Sound Pro's can you give tips on how to fix this Audio File? Email - Trick@lvcm.com"

Any help on cleanup (what software/settings to use), examples , web links on fixing this sorta of stuff is GREATLY appreciated!!!! Heck if someone wants to clean it up themselves and email it back with the steps you used to do it, that would even be better.

This advice would be Pricessless, thanks again in advance to anyone who can help this audio newbie.

Email - trick@lvcm.com

Thanks,

Trick

PS - I have read about the CEDAR system, is this hardware based only???

PSS - Cedar is a company that does audio-restoration, you better have some really valuable tapes if you want to take it to them. Lets just say you don't phone them up with some Hendrix tape you found in the garage (unless of course it was one of those unreleased tapes!). You might get a bulk deal if you secure the loan from the bank :-)

Some thoughts on this

1) I had some tapes which I wanted to convert (can't live without that Platinum Blonde!) I found it easier to download them off some file-sharing places, it took awhile to find people who had a cd rip (and who did it well - no 96k XING rips) and replace some tapes that way. Its a lot easier to get a cd-rip then to mess around with restoring them (since you don't have the master tape you can't remaster the music), and considering that even on a brand new tape your S/N ratio will be less than that of a CD your fighting an uphill battle to get the original music.

2) Probably you have some tapes that you will not find online, for those you can encode and start trying to restore them. I guess it depends on how many nits you wish to pick but I can make a suggestion - the SBlive (from experience) is not a great encoder if you've got the extra cash you might want to invest in a real 96/24 bit card.

3) Depending on how dirty you want to get your hands in restoring the sound, they have programs from "easy" - 1 click to much more control like Wavlab or Sound Forge - I might suggest Steinberg Clean as a middle-ground solution, but I'm no expert - I recommend you get some more opions for this or do some more research.

1-I should first unscrew the cassette case and put the tape into a TDK MA case (or another good case), because I've found that my Teac V2030S deck have much less flutter with TDK MA cases than, for example, TDK SA. But I usually skip this. Anyway, the tapes were already recorded with flutter.

2-I know that some of my tapes were recorded on a deck with misaligned heads. I then put off the front of the deck, and screw the playback head while the tape is running until the sound is best (in this case, remember the old position in order to go back when the recording is done).

3 - Find the exact speed at which the tape should be played, looking for CDs with one track that was also recorded on the same cassette recorder at the same time. The speed correction needed to match the wav recorded from the tape and the wav ripped from the CD must be applied on the recording. I use a resampling function for this. Say that I must set the speed to +2 % in order for the tape recording to have the same pitch as the CD. I resample then all the tapes recordings from the same recorder to 44100/1.03=42816 Hz, then reset the sample rate to 44100 in order to correct the pitch.

4 - I load all wav files in Samplitude 2496, then match volumes of each one (I sometimes need to compress dynamics), then the treble level. I use a parametric equalizer for the treble correction.

5 - I burn.

I never remove the hiss. My old tests with SoundForge noise reduction plugin were disappointing.

Sorry Pio2001 - I didn't mean to imply that switching to a 96/24 would improve the quality for the extra bits - I meant that the SBLive is not a great card for encoding since it produces a lot of noise - there is a documented problem with my old motherboard (an ASUS A7V133) where the SBLive produced so much noise that it was causing interference with the VIA chipset and resulted in data corruption. So I think not many people would say its a great choice for encoding if it adds significant noise to whatever your recording. I believe the newer SBs (Audigy2 - ie 96/24) have significantly better S/N characterisics than the SBLive.

@ cupelix - As I said you probably won't be able to replace all your cassettes that way, but most of the time its a lot faster to download some songs then then to do a Cassette-->mp3, mpc, etcAs for the mixes, i presume they are just a collection of songs, is it not possible to download the songs and re-mix them? You can even pop them into wavlab to cut it at the same spot where it runs out of magnetic media on the cassette (Some songs I can only remember that way

This topic is fascinating to me. I've done several cassettes, and I know how difficult it is. I can make only general suggestions here. Avoid digitizing at too high a level. Give yourself plenty of headroom equalization and cleanup. If you have access to any sort of analog noise reduction system try that. I've found that just a little analog and just a little digital noise reduction is the best way to go. As far as equalizing, I had some tapes with nothing on them above 12 khz. Trying to boost treble frequencies that aren't there will cause a world of pain. Best bet in this case is a harmonic synthesizer. The number one thing to do is make the best digital copy you can and save it. You may find better processing tools later on, so save that unprocessed copy.

1-I should first unscrew the cassette case and put the tape into a TDK MA case (or another good case), because I've found that my Teac V2030S deck have much less flutter with TDK MA cases than, for example, TDK SA. But I usually skip this. Anyway, the tapes were already recorded with flutter.

2-I know that some of my tapes were recorded on a deck with misaligned heads. I then put off the front of the deck, and screw the playback head while the tape is running until the sound is best (in this case, remember the old position in order to go back when the recording is done).

3 - Find the exact speed at which the tape should be played, looking for CDs with one track that was also recorded on the same cassette recorder at the same time. The speed correction needed to match the wav recorded from the tape and the wav ripped from the CD must be applied on the recording. I use a resampling function for this. Say that I must set the speed to +2 % in order for the tape recording to have the same pitch as the CD. I resample then all the tapes recordings from the same recorder to 44100/1.03=42816 Hz, then reset the sample rate to 44100 in order to correct the pitch.

4 - I load all wav files in Samplitude 2496, then match volumes of each one (I sometimes need to compress dynamics), then the treble level. I use a parametric equalizer for the treble correction.

5 - I burn.

I never remove the hiss. My old tests with SoundForge noise reduction plugin were disappointing.

MA or MA-XG ?

IIRC, TDK MA used the same casing as TDK SA ... I would use an MA-XG case (remember that cast-iron frame ...)

Aligning (and cleaning, even on a brand new tape deck) the head should bring the most audible improvement ... misaligned heads will lose much treble and will implement ugly phase errors due to azimuth problems ...

Dolby circuits work as a compressor during recording and expand during playback so be sure that you choose the correct playback setting (B/C/S) ... this could be performed within a wav editor as well, but needs some more experience ...

I would record on single wav file per cassette side and listen to it ... pitch correction can be performed if necessary, parametric or paragraphic equalisation is mostly needed. A "subsonic" filter can be applied to remove low-frequency tape hiss ...

De-Hiss should be (if carried out at all) done very carefully ... Sonic Foundry's DirectX Noise Reduction allows to listen to "residual output" ... you can listen to what is removed and will be able to tell if de-hiss affects the music ...

As far as compressing dynamics and boosting trebile I will give this a definate try. That might help with the distortion???

Boosting treble helps, old tapes usually lack treble. Dynamics compression doesn't help at all. I used it because I sometimes find on CD a track that is in my cassettes, and I end up mixing cassettes recordings with CD rips. In this case, I might need to comrpess the cassette in order to get it as loud as the CD without clipping it.

QUOTE (JeanLuc @ May 3 2003 - 11:37 AM)

MA or MA-XG ?

An MA-XG won't hurt. But in 2000, when I still recorded on cassettes, the MA had a case labeled "super precision anti resonance cassette mechanism" while the SA didn't. The flutter affecting the Bias 10 kHz calibration tone was indeed lower on the MA tapes. I can still measure it, if anyone needs it.

QUOTE (JeanLuc @ May 3 2003 - 11:37 AM)

Aligning (and cleaning, even on a brand new tape deck) the head should bring the most audible improvement ...

Yes, but the problem is the lack of reference. It is easy to set the alignment right for a given tape, but how to set it back after this, in order to record new tapes ? The only reference I could find was pre-recorded tapes. And some of them had so weak a buffer that the tape was not properly pressed against the head. So be careful, if you turn half a revolution clockwise, remember to turn half a revolution counterclockwise after the recording.Having a reference for speed is easier, because TDK tapes display their lenght in meters. I then calculated, granted that the tape is 135 meters, and the speed 1+7/8 ips, that a 90 minutes TDK tape should run 47 minutes and 15 seconds per side. Exeeding the rated lenght allows them to guarantee 90 full minutes even on mistuned deck (the tolerance is +/- 2 % for consumer gear and +/- 0.5 % for professional gear, IIRC, or maybe there is a 1% in them...)

QUOTE (JeanLuc @ May 3 2003 - 11:37 AM)

Dolby circuits work as a compressor during recording and expand during playback so be sure that you choose the correct playback setting (B/C/S)

Dolby C and S can be applied on recent tapes, granted the head is properly aligned. Old tapes loose treble, and the decoding process doesn't match the encoding process, so that the sound is "pumped" when dolby is set on playback as well as on record. Playing an old Dolby tape without Dolby doesn't harm much. It even helps to recover some of the lost treble. Dolby B is very sensitive to this problem.

I dont know if any of you bothered to download the .wav from the originalpost I made, you would tell it was HORRIBLE. Well someone actually took a crack at it and sent me back a sweet sounding file. He also listed the steps he did (although when trying to duplicate them I had no success, need more in depth/hold my hand info). Anyhow here is what he wrote. His name is left out to protect privacy.

QUOTE

Here's what I did with the file.

1. I used Waves LMB (a multiband compressor/expander) as an expander to tame down the noise. It basically works like a multiband noise gate, but without a gate!

2. It still wasn't quite clean enough, so I then processed it with Waves X-noise.

3. I then used Waves LMB again, but this time as a multiband compressor to get the basic balance between the instruments where I wanted to hear them.

4. I set up an auxillary bus with a VERY short reverb with a 30ms predelay to open up the sound just a bit.

5. I used the Waves L2 limiter at the end (on the master bus) to get the sound as "in-your-face" as possible.

Let me know what you think of the results.

That said, the original transfer isn't that great. I've done some amazing things from cassette masters.

I want to have a go with your file, but I also recieve a not found error. I want to have a go at improving the quality and find some good practices as I have a bunch of excellent drum'n'bass live tapes... most new, but some are 2nd and 3rd generation tapes, so the quality goes downhill.

If you could get some more info from the unnamed source, for example some clarification as to what settings he used and why, that would be a great help When you get the file back up, I'll take a little crack at it.

Btw - my tape deck doesn't have these B/C/S Dolby settings you speak of... it just has Dolby NR on or off.. I'm not sure which is best - on reduced the noise a lot, but seems to do so by slapping a smooth highpass filter on it, which takes out some of the high freqs...

At a guess, your first method of increasing the high frequencies will sound nasty because... if they are originally cut down to the point there is little information left, all you are doing is amplifying rubbish, giving louder sounding rubbish in the end

First, I'd agree that you should leave some headroom. I've left at least 2 dB at the peak of the loudest track when pushing it close but being careful by pre-auditioning, and 5 to 6 dB is safer in case you find an unexpected peak. Usually tape hiss will be louder than noise from your soundcard inputs and ADC (assuming you mute/deselect noisy unused inputs in your Volume Control panel Properties/Recording), so there's no point risking clipping to improve signal-to-noise ratio by 0.1 dB. Even a cheapo "PnP 16v" soundcard I used has about -60 dBFS input noise floor, which is better than a tape with Dolby.

Yes, it's usually type B for commercial recordings on cassette tapes.

This is roughly how Dolby B NR works:

The frequencies above about 10 kHz can undergo heavy dynamic compression without introducing audible harmonic distortion (because the harmonics are above 20kHz and inaudible, and are filtered out anyhow). So Dolby boosts the quieter high-freq sounds to higher levels when recording the tape, while reducing those that are already loud by a smaller degree so that they are mostly well above the level of the hiss that's inherent in the tape and mostly at >10 kHz (partly because of the speed of cassette tapes being low to extend playing time on a compact cassette).

When you play back on ordinary equipment, the treble sounds particularly bright, but hiss isn't reduced. Because the quiet sounds are made louder, cymbals and hi-hats can often sound extended in their decay and the timbre (harmonic overtones) of some instruments is overly bright compared to the original. To some, subjective quality is high (they like this treble-boosted sound), but in fact fidelity is low. The fact average people liked this modified sound allowed Dolby-B mastered cassettes to be sold by default by record companies. Even without Dolby, you could reduce hiss and maintain a pleasant sound by turning down the treble slightly.

Turning on Dolby-B applies dynamic range expansion that is the inverse of the compression applied during recording, again, only applying it to those high frequencies. This brings boosted low-level signals back to their correct low levels and at the same time, reduces tape hiss (which starts at low level) down to a much lower level, reducing this source of unwanted noise. Any harmonic distortion introduced at this time is above 20 kHz so is filtered out and inaudible anyway. Now, the cymbals and hi-hats will sound less extended in their decay, much as they ought to sound, and the timbre of instruments will be more like the original sound and quiet passages will have much less high-frequency hiss. Regardless of noise, it's the high-fidelity of transient decays and recovery of the correct musical timbre that leads me to use Dolby-B NR if the source was recorded that way.

Additionally, Dolby made demands on the acoustic quality and specs of equipment that bore their logo, so it usually assures a decent frequency response and reasonably low noise electronics, for example.

When I was into cassettes I was a student on a tight budget, and my best audio cassette player remains a Sony WM-36 Walkman with Dolby B, not part of a separates system. With the volume below maximum, it has low distortion, and with the EQ all set at 0 dB it has pretty low noise - probably about -55 dBFS on ferric mastered tapes. I have recordings with apparently legitimate 16 to 17 kHz frequencies present at the sort of places one would expect (transients, cymbals etc) and have made some pretty decent-sounding restorations. (PM me for a link to a website to listen to one before and after and see the technical details of restoration and software used). That deck's big failing is it won't play some cassettes with high-friction mechanisms at a constant speed, so in some cases I'd have to break them open and put them in a good cassette housing (which I've never tried yet) to use that tape deck.

Like Pio, I'm also cautious of digital noise reduction, having tried it in CoolEdit96 some years ago (mostly with broadband FM receiver noise, which is trickier than tape hiss when it's at a high level) and noticing some artifacts both in boundary-effects (FFT smoothing/windowing) and in tinkle. However, this may have a lot to do with CE96's implementation and something to do with the loudness and character of the noise and the amount of reduction I wanted to apply.

I've recently tried Exact Audio Copy - Process WAV - Noise Reduction. This has a 0 to 48 dB scale of reduction, not a knife-edge setting of 0 to 100% or more than 100%, which seems to respect the statistics of gaussian noise better and be easier to set to a not-too-aggressive level. I found it worked remarkably well with about 24 dB reduction with fairly critical listening. 24 dB was enough for me, so I didn't push any further.

I think I'll try to set up a blind test on this forum soon so that people can compare different software and settings and ABX them effectively.

For the blind test I'd supply a few minutes of noise from a commercial tape with long lead-out, sampled at 44.1 kHz, 16 bit on a fairly cheap soundcard of about 1999 vintage "PnP 16v". I'd also supply a separate 3 second sample from the lead-in of the same tape from which to feed the noise reduction routine with a sample of noise-only (tape hiss remaining after Dolby B NR + soundcard noise). Testers could then mix-paste the noise with CD music of their choice (preferably with <100% peaks to avoid clipping) and try to ABX the difference between original WAV and the WAV that had the hiss added then Noise Reduction applied. We could then try to determine which kinds of sound are most degraded by these routines, and which settings and software packages are considered safe, if any.

Perhaps we can also suggest improvements (e.g. apply less NR if the noise is psychoacoustically masked by the music), though I suspect that it will always be difficult to avoid degradation during fade-outs as the signal approaches the noise floor when viewed in the frequency-domain (it can theoretically fall below the apparent noise floor on waveform view and still stick far above the noise on the spectral view). I tend to think that noiselike signals (broad spectrum, like cymbals, snares, hi-hats, vocal sibilants), rather than continuous tonal signals, may be the first to get damaged as the volume decreases towards the noise floor. I have some interesting jazz samples with brushed snares that could be demanding tests.

Anyway, any of you can feel free to send me a PM if you want a link to a website to hear and read about my recent restoration which includes 'before' and 'after' samples of EAC's noise reduction on a very quiet lead-in to a quiet track only ever released on cassette. I'd welcome criticism from any of you. I had to compensate for background noise when auditioning the settings by using quiet clips and boosting the signal, so I'm not completely happy to assume I'd never notice a probem.

An MA-XG won't hurt. But in 2000, when I still recorded on cassettes, the MA had a case labeled "super precision anti resonance cassette mechanism" while the SA didn't. The flutter affecting the Bias 10 kHz calibration tone was indeed lower on the MA tapes. I can still measure it, if anyone needs it.

~~

Yes, but the problem is the lack of reference. It is easy to set the alignment right for a given tape, but how to set it back after this, in order to record new tapes ? The only reference I could find was pre-recorded tapes. And some of them had so weak a buffer that the tape was not properly pressed against the head. So be careful, if you turn half a revolution clockwise, remember to turn half a revolution counterclockwise after the recording.Having a reference for speed is easier, because TDK tapes display their lenght in meters. I then calculated, granted that the tape is 135 meters, and the speed 1+7/8 ips, that a 90 minutes TDK tape should run 47 minutes and 15 seconds per side. Exeeding the rated lenght allows them to guarantee 90 full minutes even on mistuned deck (the tolerance is +/- 2 % for consumer gear and +/- 0.5 % for professional gear, IIRC, or maybe there is a 1% in them...)

~~

Dolby C and S can be applied on recent tapes, granted the head is properly aligned. Old tapes loose treble, and the decoding process doesn't match the encoding process, so that the sound is "pumped" when dolby is set on playback as well as on record. Playing an old Dolby tape without Dolby doesn't harm much. It even helps to recover some of the lost treble. Dolby B is very sensitive to this problem.

Hmmm ... AFAIK, the TDK SA showed that SPAR-casing, too (at least until 1996 when I stopped using Compact Cassettes as a medium) ... might be that TDK removed the original SPAR-casing from the SA series for price reasons, though. TDK SA-X and MA-X had a casing named SPAR-II which was believed to be even better when measuring flutter and overall phase stablility ...

~~

When it comes to aligning the heads, a useful method is to record a perfecly correlating (L=-R) white or pink noise (can be created via cool edit and then be burned to CD) to a good and phase stable cassette before screwing the playback head for a specific recording ... for switching back to the old position, simply play the tape, put the amp to monaural output and you will hear high frequencies on a misaligned deck ... now turn the screwdriver until the high frequencies are no longer heard ... this is the position in which the reference tape has been recorded before.

~~

IIRC, dolby circuitry needs a perfecly calibrated tapedeck as well to work correctly (calibration is 10k BIAS and record level @ 315 Hz) ... if you play a tape that has been recorded with dolby and switch dolby off, you will hear more treble but less bass (due to compression during recording) ... remember to take care of this by using a parametric EQ afterwards ...

When it comes to "pumping" sounds (most likely on old cassette tapes), it definitely is better to switch dolby off completely ...

I think the same name has been applied to different tapes in different countries, and at different times. I can also remember three different brands going from good to poor over the years, without changing the name of the tape.

Re: Dolby B. You can be sure, especially if it's a pre-recorded cassette, that there's less high frequency information coming from the tape than the Dolby circuits are expecting. There was a tape deck a few yers ago which allowed you to tweak the frequency reponse before the Dolby circuits to compensate for this. Very useful in theory, but it never worked as well as I hoped in practice.

IIRC there have been various discussions Re: digitally simulating dolby de-emphasis over at the Cool Edit forums, and none of them got anywhere.

I usually have Dolby on, but boost the high frequencies afterwards. Depends on the tape really.

Getting the azimuth right is 100x more important than everything else, as has already been mentioned.

As far as compressing dynamics and boosting trebile I will give this a definate try. That might help with the distortion???

Boosting treble helps, old tapes usually lack treble. Dynamics compression doesn't help at all. I used it because I sometimes find on CD a track that is in my cassettes, and I end up mixing cassettes recordings with CD rips. In this case, I might need to comrpess the cassette in order to get it as loud as the CD without clipping it.

I've had very poor results when boosting treble... using an eq, the SN relation mantains and you end boosting the hiss of the tape.You may use some active noise filters (I've tried the filters that CoolEdit Pro 2.0 has), but those cost money and I never managed it to give good results unless the source was really REALLY bad.

With "common" (= not extremely bad) tapes I prefer not to alter the sound in any unavoidable way.

First use NO pre amp or amp. Tape directly into card or what ever is the DAC. The reason not to use the amps is they ad a tiny bit of there own noise. SO I personally prefer to go "raw" then you have that original to play with.

Also MAKE sure the tape heads are clean. Use Isopropyl Alcohol 99% so it leaves no residue.

In the sound program you can easily increase the volume. Also you can enhance what ever frequency range you desire.

I suggest use the equalizer settings. Then like you would on a stereo just play around until it sounds correct.

First use NO pre amp or amp. Tape directly into card or what ever is the DAC. The reason not to use the amps is they ad a tiny bit of there own noise. SO I personally prefer to go "raw" then you have that original to play with.

Just to clarify, you are not suggesting to take the signal directly from the tape head into the ADC. The tape deck has a built-in preamp whose output is perfectly compatible with the line in of a sound card.

As neither tapes decks nor soundcards universally adhere to the de-facto standard of 0dB FS = 2Vrms, then if the output of the tape deck is too high for the input of the soundcard, you need something between tape deck and soundcard to avoid clipping (unless the sound card lets you properly adjust the levels - many do, but some pretend to lower the level but still distort/clip the signal then just digitise the distorted signal at a lower level!).

If it's just a case of the output of the tape deck being a little too low for the input of the sound card, and the sound card is half decent, then I agree with raynault: no point introducing an additional analogue gain stage. Digital gain after capture will be good enough, and usually better.

If you think about it, it's strange that decent turntables rarely have built-in pre-amps, but cassette decks always do. Hence most integrated amplifiers expect to receive line-level with flat EQ from a tape deck, and a much lower level with RIAA EQ from a turntable cartridge. It wasn't always like this. I have an integrated amplifier from 1961, and it has low level inputs for signals direct from a phono cartridge and a tape head, plus various different equalisations that can be applied in either case. I'll never forget connecting a modern tape deck to the tape head input! TAPE (COMP) was the input from a head, and TAPE (FLAT) was the normal line level input. I didn't get it wrong again.

There is nothing in the signal path to vary the level between my phono cartridge and the soundcard input. The phono peramp is in receiver, so nothing very unusual. I've been fortunate that this works well. On a very few LPs the maximum input has been just short of 0dBfs but it has never clipped. The more than 700 LPs I transfered have been from many genera but few were Rock and none the more recent heavy noise type productions, so I don't know what those would be like.

I've done cassettes on six or ten different cassette decks, varying from probably not much more than a $100 dubbing deck to a decent quality $450 three head deck. All had the standard arrangement of a tape preamp in the deck. Every tape deck produced a significantly lower output than the average LP from my TT.

I always use a line level preamp in the signal path but since its self noise level records at about -90dBfs, it seems pointless to worry about it mucking up the signal. Its gain is 10dB. The input to its gain stage can be attenuated with the volume control. The last cassette, just last week, seemed to be in very good condition. Its wasn't exactly typical but not extreme either. Its maximum peaks, with the preamp letting through everything, were almost 15dB down. Why should my experience be so different than everyone else's?

IIRC there have been various discussions Re: digitally simulating dolby de-emphasis over at the Cool Edit forums, and none of them got anywhere.

Is the problem that the algorithm itself is poorly documented, or is the problem that a hardware circuit have access to absolute levels etc that are hard to get right in an external digital simulation?

I would think that re-implementing a 70s circuit like that in MATLAB should be easy if proper documentation is available. I don't know what improvements to expect, though.

Might be interesting to "train" the software Dolby decoder by comparing some real-world cassette output to that of a CD assumed to be based on the "same" master. Or "train" it based on the cassette output response to a known stimuli recorded to tape 2 minutes earlier (using Dolby encoding).

IIRC there have been various discussions Re: digitally simulating dolby de-emphasis over at the Cool Edit forums, and none of them got anywhere.

Is the problem that the algorithm itself is poorly documented, or is the problem that a hardware circuit have access to absolute levels etc that are hard to get right in an external digital simulation?

No, the problem is that you're replying to a post of mine from 10 years ago I've since found a solution...http://www.hansvanzutphen.com/tape_restore_live/...though I like your training idea. You don't need a specific CD though - just record test signals to a tape, which is what that guy did. The response of Dolby en/decoders is surprisingly complicated and AFAIK not perfectly documented.